1. Field of the Invention
The present invention relates to a process for producing high fructose corn syrup from glucose using noble gases.
2. Description of the Background
Wild honey is the oldest sweetener known to man, however the use of cane sugar as a sweetener dates back at least 8,000 years to the South Pacific. The sweetness of cane sugar approximates that of honey.
It was determined in 1744 that the sugar isolated from sugar beets is identical to the sugar derived from sugar cane. Thereafter, sucrose, manufactured from cane or sugar beets, being much more abundant than honey, became the sweetener of commerce. This position remained unchallenged until the development of high fructose corn syrup, a corn sweetener.
The development of corn sweeteners dates to 1811, when it was discovered that starch yielded a sweet substance when heated with acid. It was not until 1940, however, that the discovery, isolation and application of various carbohydrase enzymes afforded many new corn syrups having a variety of syrup properties. See L. E. Coker and K. Venkatasubramanian (1985). Starch Conversion Processes. Ch. 29, M. Moo-Young (ed.) Comprehensive Biotechnology, Vol. 3, H. W. Blanch, S. Drew and D. I. C. Wang (eds.), Pergamon Press, New York, N.Y., pp. 777-787.
More recently, glucose isomerase, which converts glucose to its sweeter isomer, fructose, was commercially developed. In the wake of this development, the enzymatic transformation of glucose to fructose was first introduced to corn sweetener production in 1967. The first high fructose corn syrup, commonly referred to as HFCS, contained 15% fructose. The manufacturing process, known as isomerization, originally involved the direct addition of isomerase enzymes to a dextrose substrate and a batch reactor. Further process improvements afforded HFCS products containing 42 and 55% fructose. Many producers of HFCS now further concentrate the fructose, using a chromatographic technique, and supply the concentrated material as a sweetener. See Verhoff, F. H. et al (1985). Glucose Isomerase. Ch. 42, M. Moo-Young (ed.), Comprehensive Biotechnology, Vol. 3, H. W. Blanch, S. Drew and D. I. C. Wang (eds.), Pergamon Press, New York, N.Y., pp. 837-859.
HFCS is now the most widely used sweetener in beverages and is used as well in the bakery, dairy, and canned foods industries. As noted above, HFCS is produced from corn, and it the most popular of the corn sweeteners. Other corn syrups contain glucose and dextrose; however fructose is the sweetest isomer. Through a process called wet-milling, the starch from corn is converted into corn syrup, and this syrup is then filtered to extract a syrup of highly concentrated fructose. There are presently three types of HFCS commonly sold in the United States: HFCS-42, -55, and -90, representing the percentage of fructose in the mixture. HFCS-55 is manufactured by blending 42 and 90% fructose, and is the highest value and most used of the three.
HFCS is attractive as a substitute for sugar because it is generally lower-priced and sweeter, thus necessitating smaller input per unit of output. Although HFCS is a liquid of comparable sweetness to sugar, it has different physical properties. Thus, HFCS cannot substitute for sugar in all products. For instance, baked products require sugar to ensure proper browning and for texture; jams and jellies require sugar to gel properly.
The soft drink industry accounts for approximately 75% of the demand for HFCS in the U.S. each year. Of the HFCS types available, HFCS-55 accounts for the bulk of the HFCS use in the beverage industry, representing 95% of all caloric sweeteners used in beverages. The other major type, HFCS-42, is more commonly used in the baked goods and dairy industry as a substitute for sugar in products in which color and texture, or gelling properties, are not affected by HFCS. Because of its high concentration of fructose, HFCS-90 is used primarily in reduced calorie foods such as jams and jellies.
A crystalline form of fructose can be produced from HFCS, but until recently the commercial price of this form was not competitive with the other versions. However, a cheaper production process was introduced in 1987, and experimental production of this form is continuing. Most of the crystalline form is currently used as part of a sweetener blend. Crystalline fructose may become a substitute for sugar in some new products, but the market appears to be limited.
Because of its low cost, corn has become the primary raw material source of starch syrups and sugars, including HFCS. Corn is processed into starch through a method called wet milling. In addition to the production of starch, the initial raw material in the HFCS production process, the wet milling process produces several valuable by-products that can be sold by the miller for a profit as well. Thus, these by-products serve to enhance the attractiveness of HFCS production by lowering the final cost to the producer of HFCS. In addition to starch, the wet-mill process also produces 13 pounds of gluten feed (per bushel of corn used), 2.75 pounds of gluten meal, and 1.6 pounds of oil. In terms of percentage of output produced, starch (the HFCS precursor) is the primary product, representing 67.2% of the output from a bushel of corn. Feed accounts for 19.6% of output; germ (corn oil precursor), 7.5%; and meal, 5.7%.
Corn starch is the initial raw material in the HFCS production process. Through a series of four major processing stages, the starch is converted to 42% HFCS. The major stages are: (1) conversion of starch to dextrose feedstock; (2) preparation of high quality dextrose feedstock for isomerization; (3) isomerization of the feedstock to fructose, the major and most significant stage in the process; and (4) secondary refining of the fructose product. If required, a fifth stage can be added in which additional refining of the 42% solution can be used to produce 55% and 90% fructose syrups. A diagram of the conventional 5-stage process is given in FIG. 1. Each Stage from that figure is discussed below.
Stage 1: First Enzymatic Step: alpha-amylase. The first stage converts the starch slurry to dextrose. The starch slurry is initially a mixture of amylose (approx 15-30%) and amylopectin (approx 70-85%). Several steps are involved in the production of dextrose from starch. First, the starch slurry is subjected to a high temperature treatment in which the starch granules burst and the starch becomes gelatinized. This gelatinized starch is then thinned by both high temperature and hydrolysis by alpha-amylase. This step produces liquid, less viscous and lower molecular weight dextrin products. This step takes approximately 130 minutes.
Second Enzymatic Step: amyloglucosidase. Following this liquefication and dextrinization, the dextrin products are in turn subjected to stepwise hydrolysis by amyloglucosidase to form a glucose syrup. This is referred to as saccharification, a continuous process which can take as long as 75 hours, depending on the amount of enzyme present. The end result of the saccharification process is a high dextrose (94-96% dextrose) hydrolyzate that is further refined in Stage 2.
Stage 2: The dextrose from Stage 1 is refined to produce a high quality feedstock necessary for the isomerization process in Stage 3. The refining process reduces the impurities such as ash, metal ions, and proteins which can impair the efficiency of the isomerization enzyme in Stage 3. In the refining process, the dextrose is subjected to a series of filtration steps to remove protein and oil. Next, the color of the liquor is removed through a series of granulated carbon columns. Then, the liquor is subjected to an ion-exchange system in which it is deionized. Lastly, the liquor is evaporated to the proper level for the next stage and treated with magnesium ions to inhibit any calcium ions that may interfere with the isomerase activity in Stage 3.
Stage 3: Third Enzymatic Step: Glucose Isomerase. This stage, in which the dextrose liquor is isomerized to high fructose corn syrup, is the heart of the HFCS process.
The isomerization stage converts the glucose to a much sweeter, and thus more valuable, fructose product. The key development that makes this enhanced value possible is the commercial development of immobilized glucose isomerase, a bound enzyme which can withstand the elevated temperatures of the process. The cost of the isomerization enzymes is a significant part of the total operating cost of the HFCS process. Thus, much research effort has been devoted to the economics of the activity of the enzyme and especially the rate of its decay.
Use of immobilized enzyme reactor systems (vs batch reactions, with their much longer reaction times) is the common form in the industry. The critical variable in this stage is the activity of the enzyme, which controls the rate of conversion of dextrose to fructose and determines the quality and fructose content of the product. This is a functional property of the enzyme itself and is modified by reaction conditions. The activity of the enzyme decays through time in a relatively regular manner, and the reactor system is designed and operated to minimize the fluctuations in activity resulting from this decay. For instance, the flow of the dextrin is continuously adjusted so that the residence time of the dextrin can increase to match the reduction in enzyme activity to achieve a constant conversion level through time. In addition, parallel reactors are used to increase operational flexibility. In general, at least eight isocolumns are operated in parallel and independently of the others so that each column can be put on- or taken off-line as needed.
The activity of the enzyme system is usually characterized by what is referred to as a half life. The half life of the enzyme is the amount of time that is required for the enzyme activity to be reduced by half. The enzyme system is usually operated for at least two half-lives and then replaced. Variables affecting half-life of the enzyme, and thus replacement costs, include the allowable variance in flow, pH, salt concentrations, temperature, dry solids content, metal ion concentrations, required production capacity of the processor, the number of isocolumns in parallel, and the average decay rate of the individual columns. Indeed, the economics of the HFCS process is generally analyzed in terms of costs per pound of enzyme utilized. The physical properties of the enzyme system determine its productivity and its half-life; these two parameters in turn affect the size of the reactor necessary to produce the desired conversion level and throughput of HFCS.
In general, HFCS producers seek to reach an optimal operation condition across several parameters including enzyme longevity and activity, flow rate and temperature.
The glucose isomerase system is the largest volume use of an immobilized enzyme in the Unites States. There are a number of commercial suppliers of the isomerization system.
Many of the suppliers sell fixed whole cells with isomerase activity, although other suppliers sell different immobilized forms, or a pure, isolated enzyme system. The enzyme system is used within the plant in a packed-bed reactor consisting of parallel columns of enzyme material. Most catalysts/enzyme systems are in particulate form (dry pellets). The systems available commercially vary as to the organism from which the isomerase is derived, the immobilization carrier, and the binding procedures used by the producer.
The most important variables affecting the design of a reactor system, and thus the productivity of the isomerization process include the enzyme loading factor, catalyst packing density, operational stability of the catalyst, or reactor half-life, transport efficiencies, enzyme contact and residence time.
The enzyme loading factor refers to the amount of enzyme (and thus catalytic activity) present in the immobilization system. This factor is influenced primarily by the immobilization process used to produce the enzyme system, and is determined by the enzyme producer. Load factor varies according to the relative amount of cells present, if the system is a fixed cell system, and the extent of enzyme inactivation, or loss of activity through the various enzyme preparation stages.
Catalyst packing density refers to the amount of enzyme complex present per unit volume of reactor. This is influenced in part by such variables as pressure and reactor configuration, whether linear bed or spiral.
Reactor half-life, the amount of time required to reduce the enzyme activity by half, depends on such factors as the bacteria or the organism used to produce the enzyme, and is also primarily a function of the particular enzyme system purchased.
Transport efficiency involves the rate of flow of substrate through the membrane system. Since it can be slower than the reaction time itself, the transport time of the substrate can greatly affect overall productivity.
Finally, enzyme contact and residence time, which contribute to the efficiency with which the enzyme operates on the substrate, is a function of reactor design (e.g., column size and flow rates).
Stage 3 can take as long as 4 hours.
Because of the biochemical kinetics of the conversion process, the primary product of Stage 3 is a 42% HFCS solution, which contains as well 52% unconverted dextrose, and 6% oligosaccharides. Further processing of the HFCS solution involves secondary refining of the 42% portion in Stage 4.
Stage 4: Color and ash are removed from the 42% HFCS through carbon filtration and ion-exchange systems. Stage 4 can also involve evaporation of the 42% solution to solids for shipment.
As discussed above, 42% HFCS is used primarily in bakery goods and dairy products. An additional stage (Stage 5) is required to convert 42% HFCS to 90% HFCS, which is in turn mixed with 42% to produce a third type of HFCS, 55%. 55% HFCS is used in the soft drink industry, and is the higher value HFCS product.
Stage 5: This stage involves the selective concentration of the 42% fructose: 52% dextrose product of Stage 4 to a higher concentration fructose product (90%) and its blending with the original 42% to produce other concentrations.
Since fructose preferentially forms a complex with cations, while dextrose does not, various purification processes utilize this difference either through chromatographic fractionation using organic resins or through inorganize resins in packed bed systems. The immediate product of Stage 5 is a Very Enriched Fructose Corn Syrup (VEFCS) with a 90% fructose concentration. This VEFCS can be used in turn with the 42% HFCS to produce a product with concentrations between 42% and 90% fructose, the most common being, as stated above, 55%.
The processes required to produce HFCS from corn starch is portrayed schematically in FIG. 2. FIG. 2 restates the 5-stage process given in FIG. 1 in terms of processing stages within the HFCS-producing plant.
Saccharification corresponds to Stage 1 of the process detailed above in Section 2, in which the initial input of corn starch is converted to a dextrose feedstock. Purification and pretreatment, in which the dextrose feedstock is refined further before the isomerization to HFCS, correspond to Stage 2. The isomerization process, in multiple reactors, corresponds to Stage 3. Finally, the post-treatment processes, in which impurities-are removed (Stage 4) and the HFCS can undergo further refinement to higher HFCS concentrations, correspond to Stages 4 and 5.
Thus, the production of high-fructose corn syrup is generally accomplished by the large-scale enzymatic conversion of corn starch to fructose. The process steps entail a saccharification step which consists of enzymatic hydrolysis of corn starch to dextrins and then to glucose by the action of amylase and amyloglucosidase followed by an isomerization step which entails passing saccharified syrup over a column of immobilized glucose isomerase resulting in the conversion of glucose to fructose.
The isomerization step is one of the primary process-regulating steps, and represents a major expense in the process. See Novo Industric A/S (1985.) Novo Enzyme Information. IB No. 175d-GB. Continuous Production of Fructose Syrup with Novo's Immobilized Glucose Isomerase, Sweetzyme Type Q. 56 pp. Novo Alle, D.K-2880 Bagsvaerd, Denmark; and Novo (1987) Novo Analytical Method No. AF 230/1-GB. Novo method for Activity Determination of the Immobilized Glucose Isomerase-Sweetzyme T. 7 pp. The activity and the stability of the glucose isomerization enzyme control the productivity of the process. The activity of the enzyme is the rate of conversion of glucose to fructose under given process conditions. The stability of the enzyme is an expression of its usable life span and the rate of decay of its activity under process conditions. A third potential, as yet heretofore undemonstrated, would be a change in the equilibrium concentration of fructose obtained under process conditions.
In view of the importance of this process, a need exists for a method by which the effectiveness and efficiency of the process may be improved, particularly in terms of enzyme activity, longevity, process equilibrium and/or flow rate.